Method and device for producing compressed nitrogen

ABSTRACT

The process and the apparatus serve to produce pressurized nitrogen by low-temperature fractionation of air in a rectification system which has a pressure column ( 4 ) and a low-pressure column ( 5 ). Feed air ( 1, 3 ) is passed into the pressure column ( 4 ). An oxygen-containing liquid fraction ( 11 ) is taken off from the pressure column ( 4 ) and fed into the low-pressure column ( 5 ). Gaseous nitrogen ( 18 ) from the low-pressure column ( 5 ) is at least partially condensed in a top condenser ( 17 ) by indirect heat exchange with an evaporating liquid ( 13 ). Nitrogen from the low-pressure column is produced as gaseous pressurized nitrogen product ( 24, 25, 29 ) at a pressure which is higher than the operating pressure of the low-pressure column ( 5 ). Liquid nitrogen ( 20 ) withdrawn from the low-pressure column is brought ( 21 ) in the liquid state to a pressure which exceeds the pressure of the low-pressure column ( 5 ). The pressurized liquid ( 22 ) is evaporated in a product evaporator ( 23 ) by indirect heat exchange with a heat-transfer medium ( 35 ) and is subsequently produced as gaseous pressurized nitrogen product ( 24, 25, 29 ). (FIG.  1 )

BACKGROUND AND SUMMARY OF INVENTION

The invention relates to a process for producing pressurized nitrogen bylow-temperature fractionation of air in a rectification system which hasa pressure column and a low-pressure column, in the process, feed airbeing passed into the pressure column, an oxygen-containing liquidfraction being taken off from the pressure column and fed into thelow-pressure column, gaseous nitrogen from the low-pressure column beingat least partially condensed in a top condenser by indirect heatexchange with an evaporating liquid and nitrogen from the low-pressurecolumn being produced as gaseous pressurized nitrogen product at apressure which is higher than the operating pressure of the low-pressurecolumn.

A process of this type is disclosed by DE 3528374 A1. Here, nitrogenproduced in particular at the top of the low-pressure column is removedas pressurized product. In addition, the nitrogen is taken off in thegaseous state from the low-pressure column, heated in the main heatexchanger against feed air and then compressed from about low-pressurecolumn pressure to the product pressure.

The object underlying the invention is to produce nitrogen at highpressure with relatively little expenditure.

This object is achieved by means of the fact that at least a part of theliquid nitrogen produced in the indirect heat exchange in the topcondenser or liquid nitrogen withdrawn from the low-pressure column isbrought in the liquid state to a pressure which exceeds the pressure ofthe low-pressure column, is evaporated in a product evaporator byindirect heat exchange with a heat-transfer medium and is produced aspressurized nitrogen product. The product evaporator can be disposedwithin one of the columns or outside the columns.

The pressure increase in the nitrogen product from the low-pressurecolumn is therefore at least partially carried out in the liquid state.The pressure increase in the liquid can be carried out by any knownmeasure, for example by means of a pump, utilization of a hydrostaticpotential and/or pressurizing evaporation in a tank. It implies a lowerexpenditure on apparatus than a gas compressor. Indirect heat exchangeis additionally required in which the low-pressure column nitrogenpressurized in the liquid state is evaporated. Nevertheless, this givesoverall a particularly economically favourable process.

In comparison with take-off of the pressurized nitrogen product directlyfrom the pressure column, the process according to the inventionadditionally has the advantage of higher product purity. In particular,in the low-pressure column, a concentration of more volatile componentssuch as helium, neon and/or hydrogen can be achieved which is decreasedin comparison with the top product of the pressure column. Preferably,in the invention all of the nitrogen product of the low-pressure columnis taken off in the liquid state from the low-pressure column or fromits top condenser.

The operating pressures of the double column in the process according tothe invention can be for example 6 to 20, preferably 7 to 16, bar in thepressure column and, for example 3 to 8, preferably 3 to 6, bar in thelow-pressure column. The top condenser of the low-pressure column isoperated, for example, with a liquid from the low-pressure column, suchas, for instance, the low-pressure column bottom-phase liquid, asrefrigerant. Reflux for the pressure column is usually produced by acondenser/evaporator, via which the top of the pressure column and thebottom of the low-pressure column are in heat-exchanging connection.

There are two preferred possibilities for the choice of theheat-transfer medium for evaporating the low-pressure column nitrogenpressurized in the liquid state.

Firstly, a gas from the pressure column, preferably anitrogen-containing fraction from an upper or central region of thepressure column, can be used as heat-transfer medium. This can be thetop fraction of the pressure column or a gas which is withdrawn at anintermediate point of the pressure column. This intermediate point issituated below the pressure column top by a number of theoretical plateswhich is up to ⅚, preferably ⅓ to {fraction (5/16)}, of the total numberof theoretical plates within the pressure column. The condensateproduced in the indirect heat exchange in the product evaporator isrecycled at least in part, preferably completely, back to the pressurecolumn and there used as reflux.

Alternatively, or additionally, a gas from the low-pressure column isused as heat-transfer medium for evaporating the low-pressure columnnitrogen pressurized in the liquid state, preferably anoxygen-containing fraction from a lower or central region of thelow-pressure column. This can be the bottom-phase fraction of thelow-pressure column or a gas which originates from an intermediate pointof the low-pressure column. This intermediate point is situated abovethe low-pressure column bottom by a number of theoretical plates whichis up to ⅚, preferably ⅓ to ⅚, of the total number of theoretical plateswithin the low-pressure column. The condensate produced in the indirectheat exchange in the product evaporator is recycled at least in part,preferably completely, back to the low-pressure column.

In addition, it is expedient if the liquid nitrogen only evaporates inpart in the indirect heat exchange in the product evaporator and theportion of the nitrogen which remains liquid is returned to thelow-pressure column. The product evaporator in this case is preferablyoperated as a falling-film evaporator. This type of evaporation makes aparticularly low temperature difference possible and thus acorrespondingly high evaporation pressure which, even when pure nitrogenfrom the top of the pressure column is used as heat-transfer medium, isonly slightly (about 0.3 to 0.8 bar) below the pressure column pressure.The circulation pump used is the pump present in any case for pressureboosting; the low-pressure column serves as flash gas separator when theportion which remains liquid is recycled.

To produce refrigeration it is conventional to subject a processfraction to work-producing expansion. In the context of the invention itis advantageous if the energy produced in the work-producing expansionis used for further compression of the pressurized nitrogen productdownstream of the product evaporator. The pressurized nitrogen productfrom the low-pressure column can thus be brought to pressure columnpressure with low expenditure and mixed with nitrogen product withdrawndirectly from the pressure column. The mixture can be used as product orcompressed to a still higher pressure. The process fraction to besubjected to work-producing expansion can be a partial stream of thefeed air, evaporated refrigerant from the top condenser of thelow-pressure column or a gas from the lower region of the low-pressurecolumn.

Usually, the bottom-phase liquid of the low-pressure column is used asrefrigerant to condense the gaseous nitrogen from the low-pressurecolumn in the top condenser of the low-pressure column. However, if inthe context of the process according to the invention, in addition tothe pressurized nitrogen, relatively pure or pure oxygen (purity greaterthan 40 mol %, in particular greater than 80 mol % or greater than 90mol %, preferably between 99.5 and 99.999 mol %) is to be produced, itis particularly expedient if a liquid fraction whose oxygen content isbetween that of the oxygen-containing liquid fraction from the pressurecolumn and that of the bottom-phase liquid of the low-pressure column,is used to condense the gaseous nitrogen from the low-pressure column inthe top condenser. This can be the oxygen-containing liquid fractionfrom the pressure column itself or a liquid produced after its expansionto about low-pressure column pressure, or else a liquid fraction whichis taken off from the low-pressure column above the bottom, but belowthe feed of the oxygen-containing liquid fraction. In this manner, apure oxygen product can be taken off in the liquid and/or gaseous statefrom the lower region of the low-pressure column, more precisely at thesuperatmospheric pressure of the low-pressure column. The refrigerantfor the top condenser of the low-pressure column nonetheless has ahigher nitrogen content than the oxygen product and thus a relativelylow evaporation temperature.

The invention and other details of the invention are described in moredetail below with reference to illustrative examples shown in thedrawings. In the drawings:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a first illustrative example of the process according tothe invention and a corresponding apparatus having a product evaporatorwhich is disposed outside the columns and is operated with vapour fromthe pressure column,

FIG. 2 shows a modified illustrative example with heating of the productevaporator by an intermediate fraction of the pressure column,

FIG. 3 shows a further variant of the example of FIG. 1 withwork-producing expansion of residual gas from the top condenser of thelow-pressure column,

FIG. 4 shows an example with work-producing expansion of a gas from thelow-pressure column,

FIG. 5 shows an illustrative example with simultaneous production ofpure oxygen in the low-pressure column

FIG. 6 shows a further illustrative example of the process according tothe invention and a corresponding apparatus having a product evaporatorwhich is disposed within the columns and operated with vapour from thelow-pressure column,

FIG. 7 shows an illustrative example having a product evaporator whichis disposed within the columns and is operated by vapour from thepressure column and

FIGS. 8 and 9 show illustrative examples with a product evaporatordisposed outside the columns.

DETAILED DESCRIPTION OF DRAWINGS

In the process of FIG. 1, compressed and purified air 1 is cooled in amain heat exchanger 2 and fed to a pressure column 4 at a pressure of 14bar (3). The rectification system additionally has a low-pressure column5, which is operated at a pressure of 5 bar and is in heat-exchangingconnection with the pressure column via a shared condenser/evaporator(main condenser) 6. A part 8 of the nitrogen taken off at the top of thepressure column is liquefied in the main condenser 6 and passed asreflux to the pressure column via the lines 9 and 10. Bottom-phaseliquid 11 of the pressure column is, after subcooling 15, throttled (12)as oxygen-rich liquid fraction into the low-pressure column 5. Thebottom-phase liquid 13 of the low-pressure column 5 is likewisesubcooled (14) and expanded (16) and then introduced into theevaporation chamber of the top condenser 17 of the low-pressure column5. In its liquefaction chamber, gaseous nitrogen 18 from the top of thelow-pressure column 5 condenses; a first part of the condensate 19 isrecycled to the low-pressure column and used as reflux there.

Another part 20 of the liquid nitrogen 19 from the top condenser 17 iseither, as shown in FIG. 1, taken off from the low-pressure column or isbranched off directly from the line 19. This liquid nitrogen 20 ispressurized according to the invention (in the example to 14 bar) in theliquid state (pump 21) and passed via line 22 through the subcooler 15to a product evaporator 23. The nitrogen 24 evaporated at a pressure of13.4 bar is heated in the main heat exchanger 2 and removed aspressurized product 25. It can, if appropriate, be further compressed 26in the gaseous state and, if desired, be mixed (29) with pressurizednitrogen 27, 28 withdrawn directly from the pressure column. In theexample, approximately 50% of the total pressurized nitrogen product 29originates from the low-pressure column 5.

On the liquefaction side of the product evaporator 23, a part 35 of thegaseous nitrogen 7 from the top of the pressure column 4 is condensed.The resulting liquid 36 is passed as additional reflux to the pressurecolumn 4. The product evaporator 23 is designed in the example as afalling-film evaporator in which only partial evaporation occurs.Nitrogen 45 which remains liquid is recycled to the low-pressure column5.

If required, a part of the liquid nitrogen from the top of thelow-pressure column can be produced as liquid product 30. The impureoxygen 31, which is produced by evaporating the bottom-phase liquid 13of the low-pressure column 5 in the top condenser 17 of the low-pressurecolumn, is removed as by-product or residual gas after heating in theheat exchangers 14, 15 and 2. It can be used, for example, forregenerating an apparatus for air purification.

Refrigeration is generated in the process according to FIG. 1 bywork-producing expansion 33 of a partial stream 32 of air. The expandedair 34 is introduced into the low-pressure column 5. The mechanicalenergy produced in the expansion machine 33 can be used for therecompression 26 of the pressurized nitrogen product 24 which isevaporated in the product evaporator 23, preferably by direct mechanicalcoupling of expansion machine 33 and compressor 26.

The process of FIG. 2 differs from this principally by the use of adifferent heat-transfer medium in the product evaporator. Instead of topgas 7 of the pressure column 4, here, a gas 35′ from an intermediatepoint of the pressure column is passed into the liquefaction chamber ofthe product evaporator 23. the intermediate point is situated about 20theoretical plates below the top of the pressure column 4, which, in theexample, contains in total 60 theoretical plates.

The gas 35′ still has an oxygen content of about 2 mol % and thus ahigher condensation temperature than the pure nitrogen from the top ofthe pressure column 4 (10 ppb of oxygen). The pressure on theevaporation side of the product evaporator 23 can be correspondinglyhigher (14 bar instead of 13.4 bar in the case of FIG. 1). Condensate36′ produced in the indirect heat exchange is recycled to the pressurecolumn 4 at a point corresponding to its composition, in particular thetake-off point (line 35′ or somewhat above)

Owing to the higher pressure in the evaporation 23, which was alreadyproduced using the pump 21, under some circumstances, recompression (26in FIG. 1) of the evaporated pressurized nitrogen 24′ to the pressurecolumn pressure can be omitted, and the two nitrogen products 24′, 27′from low-pressure column and pressure column can be mixed as early asupstream of the main heat exchanger 2 (line 29′).

If the double column is operated at a sufficiently high pressure (forexample 8 to 15 bar), all of the feed air 3′ can be passed into thepressure column 4. A process of this type is shown in FIG. 3, again onlythe differences from FIG. 1 being described in detail. The operatingpressures in pressure column 4 and low-pressure column 5 are, in thisexample, 15 bar and 5 bar, respectively. Process refrigeration isgenerated here by work-producing expansion of vapour 31, 31′ from theevaporation side of the top condenser 17 of the low-pressure column 5.If required, the expansion machine 33′ can, as in FIG. 1, likewise becoupled to a compressor 26 for nitrogen product.

The process of FIG. 4 is also applicable at lower pressures (example:pressure column 10 bar, low-pressure column 3 bar). Here, the expansionmachine 33″ is operated by a gas 37/38 which is withdrawn from the lowerregion of the low-pressure column 5, in particular directly above thebottom. The pressure of this gas (4.5 bar) is markedly higher than thepressure on the evaporation side of the top condenser 17 (1.25 bar). Theexhaust gas 39 of the expansion machine can be heated in a separatepassage of the main heat exchanger 2 and withdrawn as by-product; theadditional passage is dispensed with if the exhaust gas is mixed withanother fraction (vapour 31 from the top

condenser 17) upstream of the main heat exchanger and the mixture 40 isheated conjointly in the main heat exchanger 2, as shown in FIG. 4.

A process according to FIG. 5 is used if, in addition to pressurizednitrogen, pure oxygen (in the example: 99.5 mol %) is also to beproduced. This variant differs from FIG. 1 by the refrigerant 13′ forthe top condenser 17 of the low-pressure column 5 being withdrawn, notfrom the bottom, but from an intermediate point, preferably from aliquid reservoir within the low-pressure column 5 which is disposeddirectly below the feed of the oxygen-containing liquid fraction 11 fromthe pressure column 4. Below the liquid reservoir which is connected tothe line 13′ there are about 50 theoretical plates, via which the liquidflowing down is enriched to the desired oxygen purity. The oxygenproduct can be withdrawn in the liquid (42) and/or gaseous (43) state.If required, a part 44 of the liquid 42 can be passed to the topcondenser 17. If the oxygen is required under pressure, oxygen 42 can bebrought to pressure in the liquid state by the known method of internalcompression and then evaporated, for example against a part of the feedair.

The process of FIG. 6 differs in a plurality of points from that of FIG.1. For example, it exhibits a somewhat different subcooling of theprocess streams, in that only one heat-exchange block 15 is shown forthis purpose. A part of the bottom product 13 of the low-pressure column5 can be produced as liquid product (LOX). A part 160 of the nitrogen 9liquefied in the main condenser 6 can be subcooled (15) and throttled(161) into the low-pressure column 5. The bottom-phase liquid 11 of thepressure column can in part (162) be passed (163) into the evaporationchamber of the top condenser 17 of the low-pressure column. In theexample of FIG. 6, the pressurized nitrogen product 24 from the productevaporator 23 is not recompressed, but is withdrawn (29) at theevaporation pressure. Refrigeration is produced here by work-producingexpansion of residual gas, by subjecting at least a part 150 of theimpure oxygen 31 from the top condenser 17 of the low-pressure column 5to work-producing expansion from an intermediate temperature of the heatexchanger 2 in an expansion machine 133. The turbine exhaust gas 151 isreheated in the heat exchanger 2 and removed as residual gas 152 or usedto regenerate an apparatus for the purification of the feed air. Themechanical energy produced in the expansion machine 133 can be deliveredto a generator or used to compress a process fraction, preferably bydirect mechanical coupling of the expansion machine 133 to a compressorwhich is not shown.

The main difference from FIG. 1 is the product evaporator 23. This isoperated on the liquefaction side with vapour from the lower-pressurecolumn. For this purpose, on the liquefaction side the productevaporator 23, a part of the gas situated above the bottom of thelow-pressure column is condensed. The resulting liquid 136 flows backinto the low-pressure column. The product evaporator 23 is, in theexample, disposed within the low-pressure column. It can be designed asa falling-film evaporator in which only partial evaporation occurs.Nitrogen remaining liquid can be recycled to the low-pressure column 5.

In the plant shown in FIG. 7, the product evaporator 23 is built intothe double column in a similar manner to FIG. 6. Here, it is situated inthe upper region of the pressure column 4. The liquefaction side of theproduct evaporator 23, similarly to the case in FIGS. 1 to 5, receives apart 35 of the gaseous nitrogen 7 from the top of the pressure column 4.

In FIG. 8, subcooler and product evaporator are integrated in aheat-exchanger block 223. In this example, a part 246 of thebottom-phase liquid 11 of the pressure column can be used for additionaltop cooling of pressure column (via valve 248) or low-pressure column(via valve 247). Process refrigeration is produced, as in FIG. 1, bywork-producing expansion 33 of a part 32 of the feed air.

As in FIG. 8, the product evaporator 323 of FIG. 9 is constructed as acounter-current heat exchanger, preferably as an aluminium plate heatexchanger. However, in contrast to FIG. 8, it is separate from thesubcooling heat exchanger 15.

Clearly, the features of the different variants of the invention shownhere can be combined with one another. In each embodiment of the processaccording to the invention and the apparatus according to the invention,in particular in all illustrative examples, conventional rectifyingplates or arranged or dumped packings can be used as mass-transferelements in the columns of the rectification system. The combined use ofdifferent types of mass-exchange elements is also possible.

The processes of the illustrative examples and the process according tothe invention in general are suitable in particular for producinghigh-purity nitrogen having a particularly low content of more volatilecomponents such as helium, neon and/or hydrogen. For this purpose, inaddition to the outlet lines for more volatile gases (not shown in thedrawings) which are arranged on the condensers 23 and 17, other measurescan be provided.

Firstly, in all of the illustrative examples, the liquid nitrogen 20which is fed to the pump 21 can be withdrawn, instead of at the take-offat the top of the low-pressure column, at least one theoretical orpractical plate below the top of the low-pressure column. For example,up to ten, preferably three to five, theoretical or practical plates canbe situated between column top and modified take-off of the liquidnitrogen 20. Even if the low-pressure column is otherwise equipped withpackings, these plates are preferably designed as conventionalrectification plates.

Secondly, a second modification can be made in the processes of FIGS. 6to 9, in which a liquid nitrogen stream (160 in FIGS. 6 and 7) producedin the pressure column 4 is delivered (via valve 161) as reflux to thetop of the low-pressure column 5. This stream can likewise be taken offfrom an intermediate point which is situated one to ten, preferablythree to five, theoretical or practical plates below the top of thepressure column 4.

What is claimed is:
 1. A process for producing pressurized nitrogen bylow-temperature fractionation of air, comprising: feeding air to apressure column; feeding an oxygen-containing liquid fraction from thepressure column to a low-pressure column; at least partially condensinggaseous nitrogen in a top condenser of the low-pressure column byindirect heat exchange with a liquid, thereby evaporating the liquid;pressurizing liquid nitrogen produced in the top condenser or liquidnitrogen withdrawn from the low-pressure column to a pressure thatexceeds the pressure of the low-pressure column; and evaporating saidpressurized liquid nitrogen in a product evaporator by indirect heatexchange with a heat-transfer medium, thereby producing a gaseouspressurized nitrogen product.
 2. A process according to claim 1, whereinsaid heat-transfer medium is a gas from the pressure column.
 3. Aprocess according to claim 2, wherein said gas is a nitrogen-containingfraction from an upper or intermediate region of the pressure column. 4.A process according to claim 2, wherein said gas is a gas from thelow-pressure column.
 5. A process according to claim 4, wherein said gasis an oxygen-containing fraction from a lower or intermediate region ofthe low-pressure column.
 6. A process according to claim 1, furthercomprising evaporating only a part of the liquid nitrogen in theevaporator; and recycling a remaining liquid nitrogen part to thelow-pressure column.
 7. A process according to claim 1, furthercomprising expanding a partial stream of air, thereby producingwork-producing energy for compressing said gaseous pressurized nitrogenproduct.
 8. A process according to claim 1, wherein said liquid for atleast partially condensing is a liquid having an oxygen content betweenthat of a oxygen-containing liquid fraction from the pressure column andthat of a bottom liquid of the low-pressure column.
 9. An apparatus forproducing pressurized nitrogen by low-temperature fractionation of airin a rectification system, comprising: a pressure column; an air feedline leading to the pressure column; a low-pressure column having a topcondenser, said top condenser having a liquefaction side that isflow-connected to the low-pressure column by a liquid nitrogen line; aline for an oxygen-containing liquid fraction leading from the pressurecolumn to the low-pressure column; a product evaporator; and a productline comprising means for increasing the pressure of liquid nitrogenwithdrawn from the low-pressure column or the liquefaction side of thetop condenser and which directs the pressurized liquid nitrogen to theproduct evaporator, thereby evaporating said pressurized liquid nitrogenand providing a pressurized gaseous nitrogen product.
 10. An apparatusaccording to claim 9, wherein a liquefaction side of the productevaporator is connected to an upper or intermediate region of thepressure column.
 11. An apparatus according to claim 9, wherein aliquefaction side of the product evaporator is connected to thelow-pressure column.
 12. An apparatus according to claim 9, furthercomprising a liquid return line leading from the product evaporator tothe low-pressure column.
 13. An apparatus according to claim 9, furthercomprising an expansion machine coupled to a compressor for the furthercompression of the pressurized gaseous nitrogen product.
 14. Anapparatus according to claim 9, further comprising a liquid line fordirecting a refrigerant from a intermediate region of the low-pressurecolumn or a lower region of the pressure column to an evaporation sideof said top condenser.
 15. A process according to claim 1, wherein saidliquid nitrogen withdrawn from the low-pressure column is withdrawn atleast one theoretical or practical plate below a top of the low-pressurecolumn.